The present invention relates in general to an apparatus and associated method for detecting light transmitted through a scattering medium. More particularly, the present invention relates to a detection system and method using time-resolved tissue transillumination. The invention may be employed for tumor detection using a time-resolved breast transillumination technique.
Optical imaging of turbid media such as the human breast has been the subject of much research activity and has seen an increase in interest since the early 1990s. This type of imaging is based on the fact that the propagation of light in a turbid medium depends on the absorption and scattering properties of the medium. Absorption results from energy level transitions of the constituent atoms and molecules in the medium. The absorption property of the medium is quantified by its absorption coefficient xcexca, defined as the probability of a photon being absorbed per infinitesimal pathlength. Scattering results from variations in the index of refraction of the different structures present in the medium. In a highly diffusive medium, scattering is quantified by the reduced scattering coefficient xcexcsxe2x80x2 defined as the probability of a photon being isotropically scattered per infinitesimal pathlength. Characteristics such as intensity, coherence and polarization of the incident light change as it is absorbed and scattered by the medium resulting in diffuse transmittance of the light. In particular, scattering causes a collimated laser beam to spread over a sizeable volume element. This complicates the imaging of a turbid medium. Special imaging modalities must be implemented to offset the detrimental light diffusion. For example, time-resolved methods use ultra-short laser pulses to illuminate the medium. The emergent light is collected by a fast detector capable of reproducing its time variation, which can provide further information about the turbid medium. A simple data processing approach in this case is time-gating, by which only the earliest part of the output light pulses is used to produce an image. Refer to article S. K. Gayen and R. R. Alfano, xe2x80x9cEmerging optical biomedical imaging techniques,xe2x80x9d Opt. and Photon. News, 7, pp. 7-22 (1996). This amounts to using only the light with the straightest trajectory through the scattering medium, thus improving spatial resolution. See the articles: J. C. Hebden, xe2x80x9cEvaluating the spatial resolution performance of a time-resolved optical imaging system,xe2x80x9d Med. Phys, 19, pp. 1801-1087 (1992) and J. C. Hebden, D. J. Hall, and D. T. Delpy, xe2x80x9cThe spatial resolution performance of a time-resolved optical imaging system using temporal extrapolationxe2x80x9d Med. Phys., 22, pp. 201-208 (1995).
The strong interest in optical imaging of scattering media stems from the need for biomedical diagnostic techniques that are safe and non-invasive. The optical properties of biological tissues are at the heart of optically based biomedical diagnostic techniques. As for the general case of a turbid medium, the manner in which light propagates through tissue depends on its absorption and scattering properties. Thus, if abnormal tissue can be said to differ from normal in its absorption or scattering of light for some physiological or morphological reason, it then becomes possible to optically differentiate between normal and abnormal conditions. A specific application is optical mammography where tumors could be differentiated from normal breast tissue on the basis of optical properties.
Mainly two types of biomedical optical imaging exist: tomography and transillumination. Tomography is typically based on a multi-point geometry involving a large number of detectors and allows the reconstruction of 3D images. Refer to the article S. B. Colak, D. G Papaioannou, G. W. Hooft, M. B. Van der Mark, H. Schomberg, J. C. J. Paasschens, J. B. M. Melissen, and N. A. A. J. Van Asten, xe2x80x9cTomographic image reconstruction from optical projections in light-diffusing media,xe2x80x9d Appl. Opt., 36, 180-213 (1997). Obtaining 3D information is an important advantage of tomography, however, measurements and reconstructions are potentially time-consuming.
Transillumination (or 2D projection imaging) refers to a scanning procedure in which each image pixel is determined from the detection of the light that enters the medium through a certain entrance area, that propagates through it and that exits over a certain detection area usually facing the entrance area. The light entering the medium is generated by an excitation light source, typically a laser source emitting a laser beam at a wavelength in the range of 700-850 nm. The absorption of light by the tissues is minimal within this wavelength range.
A typical apparatus for obtaining transillumination images is illustrated in FIG. 1. This system typically includes an excitation light source 10, an input fiber 12, the scattering medium 14, an output fiber 16, and the detection system 18. FIG. 1 also shows at 20, the scanning direction.
The gray zone 22 in FIG. 1 indicates the volume through which the photon travel before emerging to the output surface, while the dark zone 24 indicates the same but for the limited fraction of photon that are detected. As shown in FIG. 1, optical fibers can be used to facilitate the scanning of the input beam and the area over which the output light is collected.
The adverse effect of light scattering can be alleviated by detecting the light in a time-resolved manner. For example, at each point of a transillumination scan, an ultra-short laser pulse (typically  less than 0.5 ns) can be injected at an input surface of the scattering medium. The light emerging from the opposite surface can be detected as a function of time, on a nanosecond range. This can be repeated a large number of cycles within a certain time integration in order to accumulate a sufficient photon statistics. The resulting time-resolved measurement typically appears as a vector of light intensity for different time values.
From each vector, a scalar must be calculated and transformed into a pixel value. A different method can be used for the calculation of such a scalar number. The sum of all the vector values can be performed and the resulting Continuous Wave (CW) image would be the same as obtained with a CW optical imaging transillumination technique. The sum can also be limited to the first time values which correspond to considering only the first arrival photons which experienced less diffusion and providing a less blurred image compared to a CW image. See the article: B. B. Das, K. M. Yoo, and R. R. Alfano, xe2x80x9cUltrafast time-gating imaging in thick tissues: a step toward optical mammographyxe2x80x9d Opt. Lett., 18, pp. 1092-1094 (1993). More sophisticated techniques can also be used as described in the article: Y. Painchaud, A. Mailloux, M. Morin, S. Verreault, and P. Beaudry, xe2x80x9cTime-domain opticalimaging: discrimination between scattering and absorption,xe2x80x9d Appl. Opt., 38, pp. 3686-3693 (1999).
For obtaining a good spatial resolution, the detection of the emerging light is typically done over a detection area which is small compared to the area from which the light emerges at the output surface. However, the detection of light over a large area is beneficial in order to increase the number of detected photons and thus the signal-to-noise ratio. A detection area on the order of 10 mm2 appears to be a good compromise. The detection system has to measure the emerging light intensity over a certain area and with a certain time resolution. Assuming that the entire time-distribution of the emerging light is of interest and that the injected laser power is 50 mW at 800 nm, the requirements of an ideal detection system are the following:
Large area (xcx9c10 mm2);
High numerical aperture (xcx9c0.4);
Fast time-response ( less than 0.5 ns);
High quantum efficiency at 800 nm (as high as possible);
High dynamic range (10xe2x88x928 to 1031 3 W/cm2).
Two main detection systems have been used in reported studies involving time-resolved detection of scattered light: streak cameras; see the following article: K. M. Yoo, B. B. Das, and R. R. Alfoano, xe2x80x9cImaging of a translucent object hidden in a highly scattering medium from the early portion of the diffuse component of a transmitted ultrafast laser pulse,xe2x80x9d Opt. Lett., 17, pp. 958-960 (1992) and photon counting systems; see the article: D. Grosenick, H. Wabnitz, and H. Rinneberg, xe2x80x9cTime-resolved imaging of solid phantoms for optical mammography,xe2x80x9d Appl. Opt., 36, pp. 221-231 (1997).
The typical characteristics of a streak camera are the following:
Detection area:  less than 1 mm2;
Numerical aperture: 0.2;
Time response:  less than 0.1 ns;
Quantum efficiency: 10%;
Dynamic range: 10xe2x88x927-10xe2x88x923 W/cm2 
and the typical characteristics of a photon counting system are:
Detection area:  greater than 10 mm2;
Numerical aperture: 0.4;
Time response:  less than 0.5 ns;
Quantum efficiency: 20%;
Dynamic range: 0-10xe2x88x9211 W
(0-10xe2x88x9210 W/cm2 for a detection area of 10 mm2),
(0-10xe2x88x928 W/cm2 for a detection area of 0.1 mm2).
A streak camera is typically not sufficiently sensitive to cover the entire range of light intensity. Furthermore, its detection area is small. A larger area would allow a better photon statistics.
Oppositely, a photon counting system has a larger detection area, but typically is of little use: the system is simply too sensitive and would saturate for typical conditions of operation. As a result, most of the time, the signal has to be attenuated before detection, which results in a loss of the photon statistics. Using a photon counting system, acquisition time per pixel of less than 100 ms appears difficult. See the article D. Grosenick, H. Wabnitz, H. Rinneberg, K. T. Moesta, and P. M. Schlag, xe2x80x9cDevelopment of a time-domain optical mammograph and first in vivo applications,xe2x80x9d Appl. Opt., 38, pp. 2927-2943 (1999).
In accordance with the present invention there is provided an optical imaging system for detecting light from an excitation light source and passing through a scattering medium at a plurality of detection positions. The optical imaging system comprises a photo-detector for receiving light from the scattering medium and having sensitivity in a range of intensity of interest, and an amplification circuit that is coupled from the photo-detector. An electro-optical source is coupled from the amplification circuit and converts the electrical signal into a secondary light signal. A camera receives the secondary light signal and has high temporal resolution. The system provides the secondary light signal with an intensity suitable for good signal-to-noise quality detection by the camera for the range of intensity of interest.
In accordance with another aspect of the present invention there is provided a detection apparatus operated in accordance with a time-resolved transillumination technique. This apparatus may be used for breast tumor detection. The photo detector is preferably a fast photo detector with a large detection area generally greater than 1 mm2 with a time response less than 500 ps. The electro-optical source may also have a time response less than 500 ps.
In accordance with another aspect of the present invention the camera is preferably a streak camera that allows recording of the time dependence of the light emerging from the electro-optical source with a time resolution less than 500 ps. The photo detector may comprise a photo-multiplier tube. The electro-optical source may comprise either a light-emitting diode or a laser diode.
In accordance with still another aspect of the present invention the system may include a primary output fiber for coupling from the scattering medium to the photo detector, and a secondary output fiber for coupling from the electro-optical source to the streak camera. The primary output fiber is larger in diameter than the secondary output fiber. The light detected at the primary output fiber is detected over a relatively large area, at least 1 mm2 and is preferably on the order of 10 mm2. The secondary output fiber is small compared to the primary output fiber whereby only a small fraction of the detection area of the streak camera is used.
In accordance with another embodiment of the present invention there is provided a multi-channel optical-detection system for detecting light initated through a scattering medium from an excitation light source. This multi-channel optical detection system comprises multi-channel photo detector means for receiving light from the scattering medium and a multi-channel electro-optical converting means for converting multiple electrical signals to separate multiple secondary light signals. A camera receives the multiple secondary light signals to provide an image of the scattering medium.
In accordance with still further aspects of the present invention the multi-channel photo detector means may comprise a single multi-channel photo detector. The photo detector may comprise a photo-multiplier tube including multiple anodes. A plurality of amplifier circuits interconnect the photo detector and the multi-channel electro-optical converting means. The electrico-optical converting means may comprise a plurality of light emitting diodes or a plurality of laser diodes.
In accordance with another embodiment of the present invention there is provided a method of detection of a transillumination image from a scattering medium. This method comprises the steps of, exciting the scattering medium from an excitation light source, followed by photo-electrically detecting the light from the scattering medium to provide an analog electrical signal. Next, is electro-optically converting the analog electrical signal to a secondary light signal. A camera is provided for receiving the secondary light signal and providing an image of the scattering medium.
In accordance with further aspects of the present method, there is included the step of amplifying the analog electrical signal between the steps of photo-electrically detecting and electro-optically converting. The detection is in a time-resolved manner. The step of photo-electrically detecting includes coupling the detected light by way of a primary optic fiber, and the step of electro-optically converting includes coupling the secondary light signal by way of the secondary optic fiber. The step of photo-electrically detecting may include coupling the detected light by way of a multi-channel photo-detector with multiple primary optic fibers coupling thereto. The step of electro-optically converting may include coupling the secondary light signal by way of a multi-channel source with multiple secondary optic fibers coupling therefrom. The step of providing a camera may include providing only a single camera accepting the multiple secondary optic fibers.